1. Field of the Invention
The invention relates to delta modulators in general and, more particularly, to delta modulators suitable for handling voice and voice band modem signals and to a digitally implemented delta modulator of the type which is also suitable for handling on a time-sharing basis a plurality of analog lines which may require delta coding in either single or multiple bit modulation.
2. Description of the Prior Art
Delta modulation has historically offered a less expensive approach for digitizing analog signals than PCM. This advantage exists primarily to the cost differences between the relatively simple analog filters used with delta modulation and the more critical Nyquist bandwidth filters required for PCM. When applied to coding analog signals for digital transmission on channels with potentially high error rates, delta modulation offers perhaps a more significant advantage than coder cost in being less sensitive to channel errors than conventional PCM. In the important coding rate range of 24 to 40 Kbits/sec. this robustness makes delta modulation potentially the most cost effective coding approach for channels where the error rates can degrade to 10.sup.-2 levels. Such an application would be satellite transmission systems where under rain conditions degraded operation with high channel error rates can be experienced.
In order to achieve suitable voice quality in the 24 to 40 Kbs coding range a delta coder like PCM must have some means of adapting its gain to the level of the analog input signal to be encoded. The design of the adaptation or companding method or algorithm greatly affects the error sensitivity of the delta code and determines which types of signals can be encoded. FIG. 1 illustrates a block diagram for a conventional delta coder and includes a comparator 1, a latch 2 responsive to a sample clock, a multiplier 3 for multiplying the output of latch 2 by a signal .DELTA. generated by a companding circuit 4 responsive to the output of latch 2 and a signal filter 5 whose output is the quantized representation of the analog input signal. The function of the companding or adaptation algorithm circuit 4 is to adjust the feedback step size such that the modulator provides the maximum signal to quantizing noise ratio SNR for large variation in the RMS value of the signal. An algorithm in common use (U.S. Pat. No. 3,699,566) performs this function by increasing the step size by a discrete amount upon detection of overload as evident by a pattern in the code (i.e. a run of N ones or N zeros) and decreasing the step size if overload has not occurred within a prescribed period of time. At the decoder the step size is increased or decreased accordingly from the same information in the code. Such an algorithm operates on differential gain in that the decoder derives gain change information from the received code. No information is available at the decoder as to the actual value of the step size at the encoder other than through the past history of gain changes which have occurred. If an error is produced in the digitally coded signal, it can result in an erroneous step size calculation at the decoder. Such errors result in distortions in the received signal and are more damaging subjectively than the additive background noise which occurs in non-adaptive delta modulators due to channel errors.
When an error is produced in the decoder step size value, an offset will exist between the encoder and decoder gains until some mechanism occurs to resynchronize the two values since no absolute gain reference is conveyed in the transmitted coded signal. Differential gains algorithms are suitable for coding speech signals because envelope gaps in the waveform are sufficiently wide and occur sufficiently often to allow both the decoder and decoder to reach their maximum gain value (minimum step size) and thus resynchronize. Since voice band modems signals have constant envelopes with no gaps they cannot be encoded with coders using differential gain algorithms. A single digital channel error could cause a sufficiently large increase in decoder gain to saturate the output signal to the receiving modem for the remainder of the transmission with no direct means of recovery.
A delta modulator which overcomes the above problems is disclosed by Betts at page 547 of the IEEE Transactions on Communication Technology, August, 1971. The disclosed modulator employs a direct gain companding algorithm. That is, the gain used in decoding the transmitted digital signal can be directly computed from the digital bit stream. This technique reduces the sensitivity of the coding process to channel errors and more importantly the algorithm allows the encoding of constant envelope signal such as occur with modem signals. The coder designed, however, is not suitable for use in a time sharing environment in which common components are time shared across a substantial number of lines.